Owning to advantages of high energy density, no memory effect, high working voltage, high output power, low self-discharge, long cycle life, high load capability, and no environment pollution, rechargeable lithium ion batteries are widely used in portable electronic products such as notebook computer, communication tools such as cell phone, military products, equipment for aerospace, navigation, and aviation applications, and transportation tools such as electric vehicles and motorcycles. Along with increasing demand for lithium ion batteries, the technical requirement for the batteries strengthens, particularly for battery safety performance. Although safety performance of the available lithium ion batteries has been improved dramatically, accidents like smoking, fire, and explosion may occur in case of improper use, such as overcharge and short circuit. The method for improving battery safety performance mainly comprises adding circuit protection and improving electrolyte safety performance. CN 1632983A discloses a electrolyte of lithium ion secondary battery with safety performance, which is prepared by adding aromatic compound and cyclohexyl benzene into common electrolyte of lithium ion secondary battery, wherein the aromatic compound can be phenyl ether, biphenyl, biphenyl ester, halogenated phenyl ether, halogenated biphenyl, or terphenyl, the addition amounts of the aromatic compound and cyclohexyl benzene are respectively 0.5-5 wt % and 1-10 wt % based on the weight of the electrolyte of lithium ion secondary battery. The battery with aluminum casing, which adopts electrolyte made from methoxybenzene 3 wt % and cyclohexylbenzene 2 wt %, generates no smoking, no fire, and no explosion when subjected to overcharge test at the condition of 85□, 3 C, and 10V. JP 2004214139 discloses that heat generation can be reduced by adding cyclohexylbenzene or derivative thereof (having 5-9 carbon atoms on ring being substituted) so as to improve safety. CN 1385918A discloses a electrolyte of lithium ion secondary battery, which is prepared by adding high-activity monomer of polymer as additive into prior electrolyte of lithium ion secondary battery, wherein the high-activity monomer of polymer can be one or more of pyridine based compound, biphenyl based compound, and carbazole based compound. The aforementioned electrolytes can improve overcharge protection of 4.4V battery system.
Although the aforementioned electrolytes can improve safety performance of lithium ion secondary batteries to certain extent, they also cause degradation of cycle performance and low temperature performance of the batteries at the same time.